In textile finishing, webs of material, e.g. for thermofixing in tenterframes, are heated to about 180.degree.. The exhaust air from such tenterframes is drawn off by means of a fan and is normally conducted into the open air via a flue. This exhaust air is considerably polluted with hydrocarbons, inter alia paraffins, and smells nasty. In addition, on account of its high temperature of about 140.degree. C., it contains considerable quantities of energy. Attempts to recover this energy in a heat exchanger have hitherto failed, since these heat exchangers become dirty very quickly and can hardly be cleaned.
To remove noxious substances in exhaust air, it is proposed in the VDI guidelines 2442 of June 1987 to heat the exhaust air in an exhaust-reheat plant by means of a burner to about 750.degree. to 900.degree. C. and then to cool it down again with a heat exchanger. In the process, the exhaust air to be purified is heated in the heat exchanger in the counterflow so that the fuel costs can be kept as low as possible. Such plants are exceptionally expensive and, apart from a reduction in environmental pollution do not produce any useful effect. The waste heat still contained in the gas can seldom be reasonably utilized economically. In addition, the gas-gas heat exchanger operating at high temperature poses considerable material problems so that a temperature of over 800.degree. C. desired per se often cannot be run.
In Melliand textile reports 8/1985, pages 603 to 604, it is proposed by Peter ter Duis to supply as combustion air to a steam boiler a portion of the exhaust air from a tenterframe. In this arrangement, the intake line of the burner is connected to the exhaust-air flue of the tenterframe. In the plant described, all the exhaust air from the tenterframe can be purified in certain cases at full load of the steam boiler. At the same time, the efficiency of the furnace can be improved, since it is supplied with hot combustion air. At full load of the boiler, additional fresh air is drawn in via the flue; at reduced load, on the other hand, the excess portion of the tenterframe exhaust air escapes into the open air via the flue. A considerable disadvantage of this plant is therefore that, during partial-load operation of the boiler, only a fraction of the tenterframe exhaust air can be purified and environmentalpollution thus continues to a considerable extent.
There are similar problems in the operation of a singer, wherein the problems of smell are here aggravated but the exhaust air is less moist and therefore has less heat content. The exhaust air from a singer contains more proportions of solid matter than that from a tenterframe.